This invention relates to permanent magnet assemblies and more particularly to self-retaining permanent magnet assemblies.
Permanent magnets are used in a variety of assemblies such as electric motor rotors, electric motor stators, loudspeakers, and the like. Although the present invention is described herein with particular reference to electric motor assemblies, it should be realized that the present invention is not so limited but instead extends to permanent magnetic assemblies for a wide variety of purposes.
By way of example, in certain electric motors and the like of the synchronous rotor type, the motors have a cage rotor construction with conductor bars for the rotor made of die-cast aluminum. Typically, rotor assemblies for these motors include a stack or rotor core of individual laminations constructed from suitable ferromagnetic material. Each lamination has a central opening and a plurality of so-called satellite openings adjacent to its outer margin. The laminations are assembled in a stack and the laminations are rotated slightly with respect to one another in the stack so that their central openings are coaxial but their satellite openings are skewed relative to one another to form skewed slots. The lamination stack or core is then placed in a suitable die-casting mold and molten aluminum is injected under pressure into the molds so as to surround the core, fill the mold, and flow through the skewed slots in the core formed by the satellite opening so as to form the conductor bars of the rotor. In addition, each lamination includes one or more magnet slots in which are mounted pre-formed permanent magnets, which are commonly in the shape of rectangular bars. These bar-shaped permanent magnets must be mounted in the magnet slots and suitably secured therein. However, if these slots are skewed, it is normally necessary to use a smaller than desired magnet to fit into the skewed slot, which results in a larger than desired air gap, or to use specially designed magnets (rather than the standard bar-shaped magnets) which increases the cost of the rotor assembly substantially. Even in those cases where skewing of the rotor laminations is not needed, it would sometimes be preferable to have various non-rectangular shapes for a magnet slot to provide the desired magnetic flux. But the use of custom-made magnets of these various shapes is not economical. Even when permanent magnets of the desired shape are available, there still exists an undesirable air gap between the magnets and the laminations because of the necessary manufacturing tolerances.